Douglas College Human Anatomy & Physiology I (2nd ed.) PDF

Unit 9: The Nervous System Unit Outline Part 1: The Anatomical and Functional Organization of the Nervous System Anatomical Divisions Functional Divisions Part 2: Nervous Tissue Neurons Glial cells Myelin Part 3: The Central Ner...

Unit 9: The Nervous System Unit Outline Part 1: The Anatomical and Functional Organization of the Nervous System Anatomical Divisions Functional Divisions Part 2: Nervous Tissue Neurons Glial cells Myelin Part 3: The Central Nervous System The Brain The Spinal Cord The Meninges The Ventricular System and Cerebrospinal Fluid Circulation Part 4: The Peripheral Nervous System Ganglia Nerves The Somatic Nervous System The Autonomic Nervous System Part 5: Neuronal Signalling Ion channels and the Resting Membrane Potential Generation of an Action Potential Propagation of Action Potentials Neurotransmission Learning Objectives At the end of this unit, you should be able to: I. Describe the organization of the nervous system and explain the functions of its principal components. Unit 9: The Nervous System | 113  II. Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron. III. Name, locate and describe the functions of the main areas of the human brain. IV. Describe the structure and explain the functions of the spinal cord. V. Describe the components of a reflex arc and explain how a reflex arc works. VI. Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS. VII. Describe the resting membrane potential of a neuron and explain how it is maintained. VIII. Explain how a neuronal action potential is generated. IX. Explain how neuronal action potentials travel down the axon. X. Explain the process of neurotransmission, and name three different neurotransmitters. Learning Objectives and Guiding Questions At the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task. I. Describe the organization of the nervous system and explain the functions of its principal components. 1. Draw a flow chart demonstrating the relationships between, and stating the main function of each of the following components of the nervous system: ◦ Central nervous system ◦ Peripheral nervous system ◦ Sensory neurons ◦ Motor neurons ◦ Somatic nervous system ◦ Autonomic nervous system ◦ Sympathetic nervous system ◦ Parasympathetic nervous system 2. Are the twelve cranial nerves considered part of the central nervous system, or the peripheral nervous system? Explain how you know. 3. Are the dorsal root ganglia considered part of the central or peripheral nervous system? Explain how you know. II. Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron. 114 | Unit 9: The Nervous System 1. Name the parts of a typical neuron and describe their functions. 2. Compare and contrast the location, structure, and function of: ◦ Neurons and glia ◦ Nerves and tracts ◦ White matter and nerves ◦ White matter and gray matter ◦ Nerves and ganglia ◦ Ganglia and gray matter ◦ Sensory and motor neurons III. Name, locate and describe the functions of the main areas of the human brain. 1. Describe the general anatomy of the brain, including the location of the lobes. 2. Where in the brain would you find the cell bodies of neurons? Where would you find their axons? Describe how you can tell just by looking at a (cut) brain with the naked eye. 3. Describe the location and function of each of the following areas of the human brain: ◦ Cerebrum ◦ Diencephalon ◦ Thalamus ◦ Hypothalamus ◦ Brain stem ◦ Midbrain ◦ Pons ◦ Medulla oblongata ◦ Cerebellum 4. What are the names of the three meninges, and where are they located? 5. What are the names of the four ventricles, and where are they located? 6. Describe the path taken by cerebrospinal fluid through the brain. IV. Describe the structure and explain the functions of the spinal cord. 1. Where in the spinal cord would you find the cell bodies of neurons? Where would you find their axons? Describe how you can tell just by looking at a (cut) spinal cord with the naked eye. 2. What are some of the functions of the spinal cord? V. Describe the components of a reflex arc and explain how a reflex arc works. 1. Describe the events that take place from the moment the knee is tapped to the moment when the leg extends during the patellar reflex, including the role of each of the structures involved. VI. Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS. 1. Compare the sympathetic and parasympathetic nervous system based on the: ◦ Physiological situation to which they respond ◦ Location and neurotransmitter of the central (preganglionic) neuron Unit 9: The Nervous System | 115  ◦ Location and neurotransmitter of the ganglionic neuron VII. Describe the resting membrane potential of a neuron and explain how it is maintained. 1. Describe the gating mechanism of ligand-gated, voltage-gated, mechanically-gated and leakage ion channels. 2. What is the typical resting membrane potential of an animal cell, and what factors contribute to it? VIII. Explain how a neuronal action potential is generated. 1. Draw a fully annotated figure plotting membrane potential vs. time as an action potential passes a specific location in an axon’s membrane. Include in your annotations labels explaining the main mechanisms that underlie each shift in membrane potential. IX. Explain how neuronal action potentials travel down the axon. 1. Compare the mechanism by which nerve impulses are conducted in unmyelinated and myelinated axons. X. Explain the process of neurotransmission, and name three different neurotransmitters. 1. Create an annotated diagram (or series of diagrams) showing how neurons communicate with each other: 2. Describe the mechanism by which an action potential travels from the cell body to the axon terminals of a neuron. 3. Describe the mechanisms that return a neuron to its resting state (resting membrane potential) once an action potential has passed. 4. Describe the intracellular events that occur in a neuron once an action potential reaches a synaptic end bulb. 5. Describe how an excitatory neurotransmitter causes an action potential to be produced in a postsynaptic cell. 6. Name at least three specific neurotransmitters: one from the cholinergic system, one amino acid that acts as a neurotransmitter, and one neuropeptide. 7. What factor(s) determines whether a neurotransmitter has an excitatory or inhibitory effect on a cell exposed to that neurotransmitter? Part 1: Anatomical and Functional Organization of the Nervous System The picture you have in your mind of the nervous system probably includes the brain, the nervous tissue contained within the cranium, and the spinal cord, the extension of nervous tissue within the vertebral column. That suggests it is made of two organs—and you may not even think of the spinal cord as an organ—but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs. Anatomical Divisions 116 | Unit 9: The Nervous System  The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is everything else (Figures 1 and 2). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the central nervous system is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal. Figure 1. Central and Peripheral Nervous System. The structures of the peripheral nervous system are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the central nervous system are not obvious from this overall perspective and are best examined in prepared tissue under the microscope. Nervous tissue, present in both the central and peripheral nervous system, contains two basic types of cells: neurons and glial (or neuroglial) cells. A glial cell is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. The neuron is the more functionally important of the two, in terms of the communicative function of the nervous system. To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a soma, or cell body, but they also have extensions of the cell; each extension is generally referred to as a process. There is one important process that every neuron has called an axon, which is the fiber that connects a neuron with its target. Another type of process that branches off from the soma is the dendrite. Unit 9: The Nervous System | 117  Figure 2. The Anatomical Organization of the Nervous System. Dendrites are responsible for receiving most of the input from other neurons. Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons. 118 | Unit 9: The Nervous System  Figure 3. Gray Matter and White Matter. A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by “Suseno”/ Wikimedia Commons) Figure 4. What Is a Nucleus? (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the central nervous system is a localized center of function with the cell bodies of several neurons, shown here circled in red. (credit c: “Was a bee”/Wikimedia Commons) These two regions within nervous system structures are often referred to as gray matter (the regions with many cell bodies and dendrites) or white matter (the regions with many axons). The colors ascribed to these regions are what would be seen in “fresh,” or unstained, nervous tissue (Figure 3). Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because axons are insulated by a lipid-rich substance called myelin. Lipids Unit 9: The Nervous System | 119 can appear as white (“fatty”) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker—hence, gray. The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the central nervous system —for example, a frontal section of the brain or cross section of the spinal cord. Figure 5. Optic Nerve Versus Optic Tract. This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central. Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the central nervous system is referred to as a nucleus. In the peripheral nervous system, a cluster of neuron cell bodies is referred to as a ganglion. The term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where the DNA is found; and it is a center of some function in the central nervous system (Figure 4). There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of nuclei that are connected together and were once called the basal ganglia before “ganglion” became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the “basal nuclei” to avoid confusion. 120 | Unit 9: The Nervous System  Table 1: Structures of the Central and Peripheral Nervous System CNS PNS Group of neuron cell Nucleus Ganglion bodies (i.e., gray matter) Bundle of axons Tract Nerve (i.e., white matter) Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the central nervous system is called a tract whereas the same thing in the peripheral nervous system would be called a nerve. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of axons. When those axons are in the peripheral nervous system, the term is nerve, but if they are central nervous system, the term is tract. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 5). A similar situation outside of science can be described for some roads. For example, you might know of a street named Canada Way in the city of Burnaby. If you travel south long enough on this road, eventually you will leave Burnaby and enter the city of New Westminster. In New Westminster, Canada Way changes its name to Eighth Street. That is the idea behind the naming of the retinal axons. In the peripheral nervous system, they are called the optic nerve, and in the central nervous system, they are the optic tract. Table 1 helps to clarify which of these terms apply to the central or peripheral nervous systems. Functional Divisions There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. Secondly, control of the body can be somatic or autonomic—divisions that are largely defined by the structures that are involved in the response (Figure 6). There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions. Basic Functions: Sensation, Integration, and Response The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for sensation (sensory functions) and for the response (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed integration or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response. The first major function of the nervous system is sensation—receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a particular event in the external or internal environment, known as a stimulus. The senses we think of most are the “big five”: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances (molecules, compounds, ions, etc.), touch is physical Unit 9: The Nervous System | 121 or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood. Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter’s team is so far ahead, it would be fun to just swing away. The nervous system produces a response on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and merocrine sweat glands found in the skin to lower body temperature. Figure 6. The Functional Organization of the Nervous System. The diagram represents the divisions of the nervous system involved in each of the basic functions: sensation (receiving and processing information from the external and internal environment), integration (comparing the sensory input with stored information and with other sensory inputs in order for the body to react appropriately) and response (most commonly, a motor command generated by the somatic nervous system or the autonomic nervous system). Responses can be divided into those that are voluntary or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). 122 | Unit 9: The Nervous System Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section. Somatic, Autonomic and Enteric Nervous Systems The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The somatic nervous system (SNS) is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells “Boo!” you will be startled and you might scream or leap back. You didn’t decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as “habit learning” or “procedural memory”). The autonomic nervous system (ANS) is responsible for involuntary control of the body, usually for the sake of homeostasis (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state. Figure 7. Somatic, Autonomic, and Enteric Structures of the Nervous System. Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract. There is another division of the nervous system that describes functional responses. The enteric nervous system (ENS) is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the peripheral nervous system, and is not dependent on the central nervous system. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion (Figure 7). There are some differences between the two, but for our purposes here there will be a good bit of overlap. Unit 9: The Nervous System | 123  Watch this Crash Course video for an overview of the nervous system! Direct link: https://youtu.be/ qPix_X-9t7E Part 2: Nervous Tissue Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function. Neurons Neurons are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible. Parts of a Neuron As you learned in the first section, the main part of a neuron is the cell body, which is also known as the soma (soma = “body”). The cell body contains the nucleus and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells (Figure 8). That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are dendrites (Figure 8), which receive information from other neurons at specialized areas of contact called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—meaning that information flows in this one direction. 124 | Unit 9: The Nervous System  Figure 8. Parts of a Neuron. The major parts of the neuron are labeled on a multipolar neuron from the central nervous system. Where the axon emerges from the cell body, there is a special region referred to as the axon hillock. This is a tapering of the cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. Because the axon hillock represents the beginning of the axon, it is also referred to as the initial segment. Figure 9. Neuron Classification by Shape. Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites. Many axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Unit 9: The Nervous System | 125 Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an axon segment. At the end of the axon is the axon terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic end bulb. These bulbs are what make the connection with the target cell at the synapse. Types of Neurons There are many neurons in the nervous system—a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron’s polarity (Figure 9). Figure 10. Other Neuron Classifications. Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong. Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 10). For example, a multipolar neuron that has a very important role to play in a part of the brain called the cerebellum is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787–1869). Glial Cells Glial cells, or neuroglia or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means “glue,” and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: “This connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.” Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future. 126 | Unit 9: The Nervous System  Table 2: Glial Cell Types by Location and Basic Function CNS glia PNS glia Basic function Astrocyte Satellite cell Support Oligodendrocyte Schwann cell Insulation, myelination Microglia – Immune surveillance, phagocytosis Ependymal cell – Creating cerebrospinal fluid There are six types of glial cells (Table 2). Four of them are found in the central nervous system (Figure 11) and two are found in the peripheral nervous system (Figure 12). For reference, Table 2 outlines some common characteristics and functions of the various glial cell types, but the specific names and roles of the glial cell types are not examinable material in this course. Figure 11. Glial Cells of the Central Nervous System. The central nervous system has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the central nervous system in several ways. Myelin The insulation for axons in the nervous system is provided by glial cells: oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system. Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together. Unit 9: The Nervous System | 127  Figure 12. Glial Cells of the Peripheral Nervous System. The peripheral nervous system has satellite cells and Schwann cells. Part 3: The Central Nervous System The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person’s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord. The Cerebrum The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the cerebrum with two distinct halves, a right and left cerebral hemisphere (Figure 13). Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The cerebrum comprises of a continuous, wrinkled and thin layer of gray matter that wraps around both hemispheres, the cerebral cortex, and several deep nuclei. A gyrus (plural = gyri) is the ridge of one of those wrinkles, and a sulcus (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the cerebral cortex (Figure 14). 128 | Unit 9: The Nervous System  Figure 13. The Cerebrum. The cerebrum is a large component of the central nervous system in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex. Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy (cytoarchitecture) of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as Brodmann’s areas, which is still used today to describe the anatomical distinctions within the cortex The results from Brodmann’s work on the anatomy align very well with the functional differences within the cortex. For example, Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well. Beneath the cerebral cortex are sets of nuclei known as basal nuclei that augment cortical processes (Figure 15). Some of the basal nuclei in the forebrain, for example, serve as the primary location for acetylcholine production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer’s disease is associated with a loss of neurons in the cholinergic basal forebrain nuclei. Some other basal nuclei control the initiation of movement. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep an urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.) Unit 9: The Nervous System | 129  Figure 14. Lobes of the Cerebral Cortex. The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions. (The names of the main sulci are provided but they are not required as examinable material in this course.) Figure 15. Frontal Section of Cerebral Cortex and Basal Nuclei. The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen). (The names of these nuclei are not required as examinable material in this course.) 130 | Unit 9: The Nervous System  The Diencephalon The word diencephalon translates to “through brain.” It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the peripheral nervous system all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum. The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle. The diencephalon can be described as any region of the brain with “thalamus” in its name. The two major regions of the diencephalon are the thalamus itself and the hypothalamus (Figure 16). There are other structures, such as the epithalamus, which contains the pineal gland, and the subthalamus, which includes the subthalamic nucleus, one of the basal nuclei. Figure 16. The Diencephalon. The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached. Thalamus The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. Axons from the peripheral sensory organs, or intermediate nuclei, synapse in the thalamus, and thalamic neurons project directly to the cerebrum. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention. The cerebrum also sends information down to the thalamus, which usually communicates motor commands. Unit 9: The Nervous System | 131  Hypothalamus Inferior and slightly anterior to the thalamus is the hypothalamus, the other major region of the diencephalon. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. The hypothalamus is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system. The Brain Stem The midbrain and hindbrain (composed of the pons and the medulla) are collectively referred to as the brain stem (Figure 17). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. Attached to the brain stem, but considered a separate region of the adult brain, is the cerebellum. The midbrain coordinates sensory representations of the visual, auditory, and somatosensory perceptual spaces. The pons is the main connection with the cerebellum. The pons and the medulla regulate several crucial functions, including the cardiovascular and respiratory systems. The cranial nerves connect through the brain stem and provide the brain with the sensory input and motor output associated with the head and neck, including most of the special senses. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem. Figure 17. The Brain Stem. The brain stem includes three regions: the midbrain, the pons, and the medulla. Midbrain One of the original regions of the embryonic brain, the midbrain is a small region between the thalamus and pons. The cerebral aqueduct passes through the center of the midbrain, such that these regions are the roof and floor of that canal. The midbrain includes four bumps known as the colliculi (singular = colliculus), which means “little hill” in 132 | Unit 9: The Nervous System Latin. The inferior colliculus is the inferior pair of these enlargements and is part of the auditory brain stem pathway. Neurons of the inferior colliculus project to the thalamus, which then sends auditory information to the cerebrum for the conscious perception of sound. The superior colliculus is the superior pair and combines sensory information about visual space, auditory space, and somatosensory space. Activity in the superior colliculus is related to orienting the eyes to a sound or touch stimulus. If you are walking along the sidewalk on campus and you hear chirping, the superior colliculus coordinates that information with your awareness of the visual location of the tree right above you. That is the correlation of auditory and visual maps. If you suddenly feel something wet fall on your head, your superior colliculus integrates that with the auditory and visual maps and you know that the chirping bird just relieved itself on you. You want to look up to see the culprit, but do not. Pons The word pons comes from the Latin word for bridge. It is visible on the anterior surface of the brain stem as the thick bundle of white matter attached to the cerebellum. The pons is the main connection between the cerebellum and the brain stem. Medulla The gray matter of the midbrain and pons continues into the medulla, also known as medulla oblongata. This diffuse region of gray matter throughout the brain stem, known as the reticular formation, is related to sleep and wakefulness, general brain activity and attention. The medulla contains autonomic nuclei with motor neurons that control the rate and force of heart contraction, the diameter of blood vessels and the rate and depth of breathing, among other essential physiological processes. The Cerebellum The cerebellum, as the name suggests, is the “little brain.” It is covered in gyri and sulci like the cerebrum, and looks like a miniature version of that part of the brain (Figure 18). The cerebellum integrates motor commands from the cerebral cortex with sensory feedback from the periphery, allowing for the coordination and precise execution of motor activities, such as walking, cycling, writing or playing a musical instrument. Unit 9: The Nervous System | 133  Figure 18. The Cerebellum. The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord. The Spinal Cord Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions. The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral foramina. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region (Figures 24 and 25). Gray Horns In cross-section, the gray matter of the spinal cord has the appearance of an ink-blot test, with the spread of 134 | Unit 9: The Nervous System the gray matter on one side replicated on the other—a shape reminiscent of a bulbous capital “H.” As shown in Figure 19, the gray matter is subdivided into regions that are referred to as horns. The posterior horn is responsible for sensory processing. The anterior horn sends out motor signals to the skeletal muscles. The lateral horn, which is only found in the thoracic, upper lumbar, and sacral regions, is the central component of the sympathetic division of the autonomic nervous system. Some of the largest neurons of the spinal cord are the multipolar motor neurons in the anterior horn. The fibers that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a meter in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometers in diameter, making it one of the largest cells in the body. Figure 19. Cross-section of Spinal Cord. The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) White Columns Unit 9: The Nervous System | 135  Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. Ascending tracts of nervous system fibers in these columns carry sensory information up to the brain, whereas descending tracts carry motor commands from the brain. Watch this Crash Course video for an overview of the central nervous system! (Direct link: https://youtu.be/ q8NtmDrb_qo) The Meninges The outer surface of the central nervous system is covered by a series of membranes composed of connective tissue called the meninges, which protect the brain. The dura mater is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac around the central nervous system. Beneath the arachnoid is a thin, filamentous mesh called the arachnoid trabeculae, which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the central nervous system is the pia mater, a thin fibrous membrane that follows the convolutions of gyri and sulci in the cerebral cortex and fits into other grooves and indentations (Figures 20). Figure 20. Meningeal Layers of Superior Sagittal Sinus. The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage. The Ventricular System and Cerebrospinal Fluid Circulation 136 | Unit 9: The Nervous System  Cerebrospinal fluid (CSF) circulates throughout and around the central nervous system. cerebrospinal fluid is produced in special structures to perfuse through the nervous tissue of the central nervous system and is continuous with the interstitial fluid. Specifically, cerebrospinal fluid circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The ventricles are the open spaces within the brain where cerebrospinal fluid circulates. In some of these spaces, cerebrospinal fluid is produced by filtering of the blood that is performed by a specialized membrane known as a choroid plexus. The cerebrospinal fluid circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood. There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the diencephalon, which opens into the cerebral aqueduct that passes through the midbrain. The aqueduct opens into the fourth ventricle, which is the space between the cerebellum and the pons and upper medulla (Figure 21). The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that cerebrospinal fluid can flow through the ventricles and around the outside of the central nervous system. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a choroid plexus. Ependymal cells (a type of glial cell; see Figure 11) surround blood capillaries and filter the blood to make cerebrospinal fluid. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes. Oxygen and carbon dioxide are dissolved into the cerebrospinal fluid, as they are in blood, and can diffuse between the fluid and the nervous tissue. Figure 21. Cerebrospinal Fluid Circulation. The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses. Cerebrospinal Fluid Circulation The choroid plexuses are found in all four ventricles. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the Unit 9: The Nervous System | 137 tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation. From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures. Within the subarachnoid space, the cerebrospinal fluid flows around all of the central nervous system, providing two important functions. As with elsewhere in its circulation, the cerebrospinal fluid picks up metabolic wastes from the nervous tissue and moves it out of the central nervous system. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that cerebrospinal fluid can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs and wastes to be filtered out by the kidneys (Table 3). Table 3: Components of Cerebrospinal Fluid Circulation Lateral Third Cerebral Fourth Central Subarachnoid ventricles ventricle aqueduct ventricle canal space Between pons/ External to upper Spinal entire central Location Cerebrum Diencephalon Midbrain medulla cord nervous oblongata system and cerebellum Blood Choroid Choroid Choroid Arachnoid vessel None None plexus plexus plexus granulations structure Part 4: The Peripheral Nervous System The peripheral nervous system is not as contained as the central nervous system because it is defined as everything that is not the central nervous system. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the peripheral nervous system, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the enteric nervous system and are a special subset of the peripheral nervous system. Ganglia A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a dorsal root ganglion. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the central nervous system through the dorsal nerve root. The other major category of ganglia, those of the autonomic nervous system, will be examined later in this chapter. 138 | Unit 9: The Nervous System  Figure 22. Dorsal Root Ganglion. The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM × 40. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) Nerves Bundles of axons in the peripheral nervous system are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. Nerves are associated with the region of the central nervous system to which they are connected, either as cranial nerves (12 pairs) connected to the brain or spinal nerves (31 pairs) connected to the spinal cord. The cranial nerves are primarily responsible for the sensory and motor functions of the head and neck, although one of these nerves, the vagus, targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve. The Somatic Nervous System The somatic nervous system is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures of the peripheral and central nervous systems and the functions of the somatic and autonomic systems can most easily be demonstrated through a simple reflex, an automatic response that the nervous system produces in response to specific stimuli. The neurons and neural pathways responsible for a reflex action constitute the reflex arc. One of the simplest reflex acts is the stretch reflex, by which the nervous system responds to the stretching of a muscle (the stimulus) with contraction of that same muscle (the response). This response protects the muscle from over-stretching, but more importantly, it has a crucial role in maintaining posture and balance. The patellar reflex (or knee-jerk reflex) is an example of stretch reflex and it occurs through the following steps (Figure 23): Unit 9: The Nervous System | 139  Tapping of the patellar tendon with a hammer causes the stretching of muscle fibers in the quadriceps muscle, which stimulates sensory neurons innervating those fibers. In the sensory neuron, a nerve impulse (action potential) is generated, which travels along the sensory nerve fiber from the muscle, through the dorsal root ganglion, to the spinal cord. The sensory neuron stimulates a motor neuron in the ventral horn of the spinal cord. That motor neuron sends a nerve impulse (action potential) along its axon. This impulse reaches the quadriceps muscle, causing its contraction and the extension of the leg (a kick). The sensory neuron can also activate an interneuron (e.g., Figure 23), which inhibits the motor neuron responsible for the contraction of the antagonistic muscle to quadriceps (i.e. hamstring). Figure 23. The Patellar Reflex. The stimulus (stretching of the quadriceps muscle caused by tapping on the tendon) triggers a nerve impulse in a sensory neuron, which synapses with and stimulated a motor neuron, leading to the contraction of the quadriceps. (credit: www.backyardbrains. com/experiments/ Musclekneejerk, protected under Creative Commons License) Another example of a simple spinal reflex is the withdrawal reflex, which occurs, for example, when you touch a hot stove and pull your hand away. This reflex occurs through a similar sequence of steps: Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. In a sensory neuron, a nerve impulse (action potential) is generated, which travels along the sensory nerve fiber from the skin, through the dorsal root ganglion, to the spinal cord. The sensory neuron stimulates a motor neuron in the ventral horn motor of the spinal cord. That motor neuron sends a nerve impulse (action potential) along its axon. This impulse reaches the biceps brachii, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove. The basic withdrawal reflex includes sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii). As seen for the patellar reflex, the withdrawal reflex can also include inhibition of the antagonistic muscle (triceps brachii in our example). Another possible motor output of the withdrawal reflex is cross 140 | Unit 9: The Nervous System extension: counterbalancing movement on the other side of the body by stimulation of the extensor muscles in the contralateral limb. The somatic nervous system also controls voluntary movement and more complex motor functions. For example, reading of this text starts with visual sensory input to the retina, which then projects to the thalamus, and on to the cerebral cortex. A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the extraocular muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles). The Autonomic Nervous System The autonomic nervous system is often associated with the “fight-or-flight response,” which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. To suggest what this means, consider the (very unlikely) situation of seeing a lioness hunting out on the savannah. Though this is not a common threat that humans deal with in the modern world, it represents the type of environment in which the human species thrived and adapted. The spread of humans around the world to the present state of the modern age occurred much more quickly than any species would adapt to environmental pressures such as predators. However, the reactions modern humans have in the modern world are based on these prehistoric situations. If your boss is walking down the hallway on Friday afternoon looking for “volunteers” to come in on the weekend, your response is the same as the prehistoric human seeing the lioness running across the savannah: fight or flight. Most likely, your response to your boss—not to mention the lioness—would be flight. Run away! The autonomic system is responsible for the physiological response to make that possible, and hopefully successful. Adrenaline starts to flood your circulatory system. Your heart rate increases. Sweat glands become active. The bronchi of the lungs dilate to allow more air exchange. Pupils dilate to increase visual information. Blood pressure increases in general, and blood vessels dilate in skeletal muscles. Time to run. Similar physiological responses would occur in preparation for fighting off the threat. This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack. In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous syst